Calcining lime bearing sludges



Aug. 25, 1953 s, wm'rg 2,650,084

CALCINING LIME BEARING SLUDGES v Original Filed June 28, 1946 2Sheets-Sheet 2 e ma a .m e Pe m m .m

S a O m 0 0 0 HI m B .6 m w EF 9m n 2|. W. e c o O MD vmd S" h C .m 8 0n0. F.mB GE 00 o o o 000 b owww o o ww o 00 m INVENTOR FRANK 3. WHITE, BY

ATTORNEY FIG.2.

Patented Aug. 25, 1953 CALCINING LIME BEARING SLUDGES Frank S. White,Stockbridge, Mass, assignor to The Dorr Company, Stamford, Conn., acorporation of Delaware Original application" June 28; 1946, Serial No.680,191. Divided and this application February 26, 1949, Serial No.78,569

'3 Claims. (01. 263--53) 1 This invention relates to the calcinationamong other things to trade sludges that are lime-bearing, such as papermill causticizing sludge, water softening sludge, sludge from firstcarbonation or other lime sludge producing stations in sugar factories,and so on.

Such sludges are currently burned or calcined in kilns or reverberatoryfurnaces but the expense thereof is high and their operation notparticularly efiicient, partly because of lack of. close temperaturecontrol both generally and locally, and high dust losses. So it is oneobject of this invention to devise ways and means for overcoming suchdisadvantages. Another object is to devise ways and means whereby suchsludges or solids and particularly precipitated calcium carbonate(CaCOa) can be calcined by the use of heated fluidized beds of thematerial being treated without the usual'high dust losses or loss offines. By fluidized bed is meant a reactor or furnace wherein the bed ofthe material being treated is fluidized or mobilized to simulate a fluidby an upwardly flowing stream of reaction gas, the bed being continuallymaintained by admission of fresh feed at the bottom and continuallydischarging treated material descendingly from the bed. The reaction gasupflowing through the bed of solids is given a velocity high enough tofluidize r mobilize the larger solids or lumps. This naturally tends toblow the fines out of the bed to loss. So it is an important object ofthis invention to minimize this latter loss and to maximize to retentionand calcination of the fines within the bed.

This patent application is a division from my patent application SerialNo. 680,191, filed June 28, 1946, now Patent No. 2,465,410, issued March29, 1949, the drawings and most of the description thereof being thesame in both applications. In the parent case, emphasis was given to thecalcining of the lime-bearing material and the nodulization orpelletization of that material prior to its discharge from the reactorby the presence in the, material of the bed of a heatresponsiveadhesive. The emphasis of this case has nothing to do with the presenceor absence of such adhesive because its end product need not benodulized at all. The emphasis is directed to the discovery that finepowdery material can be calcined by fluidization processes without theusual prohibitively high dust-losses if the expedient is used of havingpresent in the fluidized bed at the time the fines are fed theret0,'asubstantial quantity of incandescent calcinable coarser solids or lumps.In other words,

I have found that if a bed of calcinable larger bed solids is fluidizedand brought up to incandescent calcining temperature, with velocity ofthe fluidizing gas sufiicient to mobilize these larger solids and thusof course high enough to blow fine solids from the bed, such fines whenfed to the bed beneath its upper surface or fluidlevel unexpectedly andunobviously will not be so blown but are somehow retained in the bedirrespective of the gas velocity being high enough for normally blowing.out such fines and they will be so retained as long as a significantquantity of such incandescent larger solids are present.

These objects are attainable by establishing and maintaining in anenclosing reactor or furnace, one or more beds Whose solids are heatedto calcining temperature and are supported above a gas-permeable plateor partition through which reaction gas is forced at a. velocity forfiuidizing the coarser solids whereby they simulate a turbulent boilingfluid. Solids are continually fed to the bed and treated solidscontinually removed therefrom. This treatment is characterized inthatthe reactor has present in it coarser solids, namely having a grainsize lying in a range of from minus 14 to plus screen mesh (Tyler) towhich is supplied fine calcium carbonate powder such as driedwater-softening sludge. The bed can then be fluidized and heated tocalcination temperature. The dried CaCQa powder .working its way throughthe bed of coarse material is rapidly decomposed to calcium oxide andcarbon dioxide gas. Upon leaving the bed, entrained solids can beseparated from the gas by a dust collector. Small additional quantitiesof coarse material may be added from time to time to compensate forattrition losses.

The fluidized coarse-and-fine bed permits the obtaining of the principaladvantages of the fluidizing process, some of which are uniformtemperature control, surface combustion, intimate gaseous-solidscontact, and rapid heat transfer,

The invention may be practiced in an enclosed reactor or furnace, moreor less vertical in type, divided by a transverse horizontally-extendinggas-permeable plate or partition, into a gasreceiving or wind-boxcompartment beneath the plate and a solids heat-treatment zone or coin--partment thereabove. A bed of solids to be treated is supported by theplate by means of an oxygen-bearing gas or air blown into and throughthe bed of solids'so that at least the solids in the upper section orzone of the bed are in mobilized non-segregating suspension in the gaswhereby they are in fluidized condition and thereby act as if theycomprised a fluid.

Fuel is supplied to the upper zone of the bed to maintain combustion oroxidation therein. Solids are fed for treatment in the reactor into theupper zone of the bed. Calcined solids are discharged from the bottomsection of the lower zone of the bed. Products of combustion aredischarged from the top section of the reactor. The reactor is operableon a continuous basis. The sludge or solids to be fed are more or lessdried and pre-heated by rising hot products of combustion from thereactor, whereupon they are conducted into the middle section of the bedin the reactor-the middle section being that toward the bottom of thesuperjacent fluidization zone of the bed and toward the top of thesubjacent cooling zone of the bed. Combustion is maintained in thefluidization zone of the bed and the temperature thereof carefullycontrolled to be just enough above the decomposition or oxidationtemperature of the sludge solids in order to effect efllcientcalcination thereof without using excessive fuel.

As the solids in the calcination zone are in fully fluidized gassuspension, calcination proceeds apace but there is a gradual controlledrate of descent or sinking of the hot calcined fluidized solidsdownwardly past the middle section of the bed into the lower or bottomzone of the bed, where due to a lesser quantity and velocity of uprisinggas therein, the descending solids are de-fluidized and cease to begas-suspended, but are capable of downward mass movement resulting indischarge thereof from the reactor because of the controlled andlessened upward velocity of gas they encounter uprising in thede-fluidization zone of the bed. The lesser quantity and velocity of airor gas uprising through this bottom zone of the bed comes thereintothrough orifices in the constriction plate which supports the compositebed.

Solids in the fluidized calcination zone are incandescent but indescending or sinking into the de-fluidization zone cool quickly becausethey radiate or exchange their heat to the uprising gas around them.Actually, the hot solids descend against a stream of uprising cool gas,so there is effected here, countercurrent cooling, whereby heat isstripped rapidly from the descending solids. Therefore by the time thesolids discharge to the atmosphere from the defiuidization zone of thebed, they are quite cool.

The best embodiment of which I am at present informed is illustrated inthe accompanying drawings, but this embodiment is shown by way ofexample only since it is possible to modify structural and operativedetails, so long as no departure is made from those required in theappended claims. In the drawings, Fig. 1 is a vertical sectional view ofan embodiment of this invention with certain accessories which arehelpful in practicing the invention. Fig. 2 is an idealized view showingentrained fines being scrubbed from a gas stream by the coarse fluidizedbed and the mixture of coarse and fine solids being discharged.

In Fig. 1 a more complete reactor is shown, usually made of refractorymaterial, and mounted on legs if desired. Above the fluidized bed I3 isa freeboard space 39 deflned at its top by a second or upper apertureconstriction plate or partition 40 comprised of refractory bricks, oneor more of which contain a funnel-shaped aper- 4 ture 4| (more or lesslike aperture II of Fig. 4) that is topped by a ball check 31 (see Fig.5). Supported from the upper face of the apertured partition 40 is apreheating bed 42 of fluidized solids having a fluid level 43 determinedby a discharge weir 44 formed by the inlet to the pipe or conduit 28.Above the bed is a freeboard space 45 and above that is the top 46 ofthereactor. 11 represents a standpipe extending vertically through thepartition 40 and the bed 42 which is controlled by a valve 48. Thisstandpipe is for the purpose of by-passing hot gas rising from thecalcining bed l3 past the bed 42 into the freeboard space 45.

From the top 46 of the reactor, there rises a stack 41 having anenlarged section 49. The stack terminates in a cyclone 50 from which gasescapes or exhausts through outlet 5| and dust or dried solids spill ordescend through a downcomer pipe 53, valved at 54 leading to a pug-mixertrough 55. 56 indicates a rotary filter of wellknown type that deliversfilter-cake into the pug-mixer 55 which re-pulps or diminutes the cakeand mixes it with dried solids spilling through downcomer 53 anddelivers the mixture to a motor-driven cage mill 5! in the enlargedsection 49 of the stack 41. From the downcomer 53, there is anextensionpipe or conduit 58, valved as at 59 that conducts dust or dried solidsto a motor-driven screw conveying feeder 6| which supplies the solidsinto the upper fluidized bed 42 but beneath its fluid level 53. Make-upcoarse limestone may be added to the dried finer solids pipe 58 throughthe make-up pipe 82 for coarse solids controlled as by a valve 63.

Cooling of the solids in the cooling bed section l5 may be facilitated,if desired, by supplying air through pipe BLValved at 58, leading to acircircular bustle pipe 69 in the reactor base that is equipped withtuyeres 10. This air in cooling the solids becomes preheated so thiscomprises an eflicient heat-saving step. Since the cooling bed l5 ofsolids is more compact than its superjacent fluidized bed l3, itrequires less volume so it is advantageous to taper the inside faces ofthe reactor inwardly toward the bottom, as shown at I I.

It is here pointed out that the bustle pipe and tuyeres are usedinitially to preheat the charge by means of hot combustion gases, to apoint where combustion of the normal fuel will take place.

The apparatus described is adapted for the calcination of lime-bearingsludges or muds or precipitates as result from industrial processes.They include water-softening sludge, paper mill sludge that isfrequently called causticizing sludge, sugar factory mud or sludge thatresults from the first carbonation station or other such lime-usingstations in sugar factories, and the like.

The starting material is fed to the drying and preheating treatment. Thepreheating is done preferably in an upper compartment in the reactor llwherein a bed 42 of such solids is maintained above a transversehorizontally-extending apertured constriction partition 40 through whichhot gases comprising products of combustion taking place therebelow,uprise in such velocity that the solids of this bed are maintained in astate of mobility or fluidization wherein they are like a fluid. The hotuprising gases rise from a calcining combustion zone that is maintainedat about 1850 F. and are more than enough to maintain the solids of thepreheating fluidized .bed at the 1000" F. desired, so a. portion must beby-passed (through pipe 11) around the bed to the contact drying step.The uprising gases en-' train inevitable some solids continually up outof the bed (however much minimized by the use of this invention) into asuperposed cyclone 50 from whence they are conducted through a pipe 58leading back to the bottom of the preheating bed ,42. But a major partof the preheated solids of bed 42 spill over weir 44 into pipe 28' andare then fed tothe calcining bed I3 of the reactor through pipe while inair suspension. This is accomplished by air being added to the solidswith such force that the solids are carried by the air under pressuredirectly into the fluidized upper zone A containing the calcination bed13 in the reactor. Here the solids encounter a calcining temperature ofabout 1850 F. which is just above the decomposition temperature of thecalcium carbonate constituents of the solids, whereupon the solids whichis mobilized suspension and fiuidization have their calcium carbonateconstituents converted into CaO and CO2, namely, they are calcined. Uponbeing, calcined, the solids sinkinto the lower or cooling zone C. Fromhere, they sink further and descend to discharge as cool end productfrom the reactor through discharge pipes I8. I

In starting up, a preliminary seeding of the calcining bed is requiredso this is done by supplyingthereto through pipe 62 to form the initialbed, coarse limestone or other calcinable solid particles or lumpsranging in size from about minus 14 screen mesh to plus 65 screen mesh(Tyler). If more incandescent lumps are needed in the bed for receivingfreshly fed fines, because of attrition of a significant number of thelumps, fresh lumps can be supplied through pipe '62 to enter thefluidized bed at an elevation beneath its upper surface of fluid-level.The valve 63 provided in pipe 62 permits such lumps to be fed from timeto time at the will of the operator, but under conditions that freshlyfed lines upon entering the bed encounter there a mass of incandescentfluidized coarse particles. In the upper zone A (bed section l3),wherein calcination takes place, fluidization of the solids thereintakes place because of the volume and velocity of gas uprisingtherethrough. Such velocity can be from 4 to 6 feet per second withoptimum being 4.5-5.0. The gas, so uprising, is made up from threecontributing sources, namely, (1) the gas supplied directly into thefluidization zone which also is a conveying medium for introducingsolids into that zone, (2) products of calcination taking place'in thatzone, and (3) gas uprising into that zone from the subjacent coolingzone C (bed section IS). The velocity of the gas uprising in thesubjacent zone may be less than that which produces fluidization of thesolids therein and yet be sufiicient to permit the solids to migratethereinto from the superjacent fluidized zone and to sink graduallycompletely through the cooling zone-to discharge. Such velocity is fromone-fourth to one-half the velocity in the superjacent fiuidization andcalcination zone.

Another feature of importance resulting from this manner of constructionand operation is due to the fact that there is present in thecalcination zone both large solids or lumps and small solids or fines.As the solids of this zone are maintained at calcination temperature,they are incandescent and being fluidized or mobilized, each largeincandescent solid is surrounded by a gas space. Fresh solids to beheat-treated in the zone are supplied thereto in finely divided if notpulverulent form, so that they pass into the spaces between two or morelarge solids or lumps which are feedable through pipe 62 as and whendesired due to the presence of valve 63. The incandescent lumps radiatetheir heat to these newly fed fine solids whereby they are quicklybrought up to calcination temperature and the in-candescence seems totrap and retain a ma jority of the finer solids that one would expectotherwise to be blown from the reactor by the velocity of the uprisingair that is sufficient to fiuidize the larger solids or lumps. Moreover,when these incandescent lumps sink into the subjacent cooling zone, theygive up their heat quickly to the gas uprising through that zone andthereby preheat it before it reaches the calcination zone, and this alsocontributes to the saving in fuel used to heat that zone.

While the invention has been described as applicable to the calcining oflime-bearing sludges or slurries, it may also be used in the burning ofcement and in the sintering of red mud (from the Bayer method ofprocessing low-grade bauxite) with limestone and soda ash.

More particularly and referring again to the drawings and especiallyFig. 1, it will be pointed out that the lime-bearing sludge or othermaterial to be calcined, is supplied to the filter 56. Filter-cake fromthe filter drops into the pugmixer trough 55 where it is repulped ordiminuted and mixed with dried solids descending into the pug-mixer fromthe cyclone 50. The repulped cake and dried solids are then acted uponby the motor-driven cage mill 51, operating in the enlarged section 49of the stack 41, whereupon they are dispersed amid and rise with therising stream of hot gases, thus being dried by contact. After dryingthe sludge is collected in cyclone 58, conveyed in part back topug-mixer 55 for feed conditioning, the remainder going to bed 42through conduit 58 and valve 59 whereupon it (the sludge) is preheatedto 1000 F. These solids are fluidized by hot gas from the freeboardspace 39 uprising through the apertures 4| in the construction partition40 in such volume and at such velocity, that (1) the ball checks 31 areunseated and thus disperse or diffuse the uprising gas, and (2) thesolids of the bed 42 are fluidized or mobilized. When the reactor isshut down, and no gas uprises through the apertures 4 I, the ball checks31 become seated and thus seal the apertures 4| against the bed 42unloading itself downwardly into the next lower bed l3. The uprising gasis hot enough to preheat these solids. Preheated solids flow from thetop of the bed or its fluid level 43 over weir 44 and spill downfeedpipe 28.

Dried solids or dust from cyclone 50 in excess of that desired to be fedto the pug-mixer 55 falls down pipe 58 to the screw feeder 6| and arethus supplied to the preheating bed 42, beneath the fluid level thereof.

Solids, thus preheated and spilling down pipe 28 are led to pipe 25where they are joined by a blast of gas or air through pipe 26 with theresult that the solids are gas-conveyed to and through the manifold body18 and thence upwardly through hollow arms l1, and finally they areemitted together with their conveying gas, out into the fluidized bedsection l3 past the unseated ball checks 31 from whence they enter thebed [3 where there is present the lumps supplied thereto through pipe62. If and when gas flows stop, the ball checks 81 seat and preventsolids -oi the bed I! from being purged through the arms H. The bedsection II and its contents are maintained at calcination temperature bymeans of burning fuel from pipes 32, either gas or liquid as the casemay be, to maintain combustion in that bed. Since the gas supplied withthe solids to be treated usually is air, there is enough oxygen presentto support calcining combustion at carbonate dissociation ordecomposition temperatures.

Incandescent calcined solids descending or sinking from the fluidizedbed I3 become de-fiuidized because the volume and'velocity of the gassupplied through pipe 30 and wind-box 24 as well as through the tuyeresI is insuflicient to fluidize. The deficiency is made up by theadditional gas added through the manifold arms I! so that the solids inthe superjacent bed l3 will be fluidized. The sinking solids give uptheir heat quickly to the gas uprising against. their coarseincandescent bed solids by supplying such coarse solids to the bed whileupflowing gas therethrough at a velocity sufllcient to maintain thesolids in a turbulently mobilized non-stratifying fluidized condition,continually supplying to the bed for treatment therein gas-entrainedfine solids, said solids lying in a size .and specific descent so thatis an eflicient heat transfer and cooling. Cooled, calcined solidsdischarge Irom the de-fluidized bed I! through discharge pipes l9.

Since there is considerably more volume of hot gas in the freeboardspace 39 made up of products of combustion and fluidizing gas, it isimportant to by-pass some of it in order that only enough gas risesthrough the constriction partition 40 to fluidize the solids inpreheating bed 42, and not much more. Such excess gas is by-passed upstandpipe 11 that is controlled by valve 48.

With respect to the several valves hereof, they have been indicated in aconventional manner. Valves adapted for meeting the required uses arewell known and should accordingly be so selected, for example, valvessuch as 59, 63, 54, 29 and 23 can well be of types known as rotary orstar valves since they execute the function of pressure locks.

I claim:

1. The process for retaining fine solids inan incandescent fluidized bedfor treatment therein under conditions for normally entraining such finesolids, which comprises establishing and maintaining a fluidized bedcontaining relatively gravity range that would normally be entrained bythe fluidizing gas at the velocity selected to fluidize the bed,discharging irom the bed a mixture of bed solids and treated finesolids, and retaining such tine solids in the bed until they have beentreated therein and subsequently discharged therefrom bymaintaining therelatively coarse bed solids to lie in a size range of fromsubstantially 14 to substantially screen mesh, whereby fine solids areretained in the bed for surface.

FRANK S. WHITE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 124,097 Taylor Feb. 27, 1872 692,257 Eldred Feb. 21, 1902714,842 Wentz Dec. 2. 1902 0,903 Rudback Jan. 27, 1942 2,300,042Caldwell Oct. 27, 1942 2,304,827 Jewell Dec. 15, 1942 2,367,281 JohnsonJan. 16, 1945 2,393,909 Johnson Jan. 26, 1946 2,409,707 Roetheli Oct.22. 1946 2,409,787 ONeal et al Oct. 22, 1946 2,465,410 White Mar. 29,1949 2,529,366

Bauer Nov. 7. 1950

1. THE PROCESS FOR RETAINING FINE SOLIDS IN AN INCANDESCENT FLUIDIZEDBED FOR TREATMENT THEREIN